Heat flow is a form of energy transfer that occurs between parts of a system at different temperatures. Heat flows between a first media at one temperature and a second media at another temperature by way of one or more of three heat flow mechanisms: convection, conduction, and radiation. Heat transfer occurs by convection through the flow of a gas or a liquid, such as a part being cooled by circulation of a coolant around the part. Conduction, on the other hand, is the transfer of heat between non-moving parts of system, such as through the interior of solid bodies, liquids, and gases. The rate of heat transfer through a solid, liquid, or gas by conduction depends upon certain properties of the solid, liquid, or gas being thermally effected, including its thermal capacity, thermal conductivity, and the amount of temperature variation between different portions of the solid, liquid, or gas. In general, metals are good conductors of heat, while cork, paper, fiberglass, and asbestos are poor conductors of heat. Gases are also generally poor conductors due to their dilute nature.
Common examples of heat exchangers include burners on an electric stove and immersion heaters. In both applications, an electrically conductive coil is typically used that is subjected to an electric current. The resistance in the electric coil generates heat, which can then be transferred to a media to be thermally effected through either conduction or convention by bringing the media into close proximity or direct contact with the conductive coil. In this manner, liquids can be maintained at a high temperature or can be chilled, and food can be cooked for consumption.
Because of the favorable conductive and convective properties associated with many types of fluid media and the transportability of fluids (i.e. the ability to pump, for example, a fluid from one location to another), many heat exchangers utilize a moving fluid to promote heat transfer to or from an object or other fluid to be thermally affected. A common type of such a heat exchanger is one in which a heat transfer fluid is contained within and flows through a confined body, such as a tube. The transfer of heat is accomplished from the heat transfer fluid to the wall of the tube or other confinement surface of the body by convection, and through the confinement surface by conduction. Heat transfer to a media desired to be thermally affected can then occur through convection, as when the confinement surface is placed in contact with a moving media, such as another liquid or a gas that is to be thermally affected by the heat exchanger, or through conduction, such as when the confinement surface is placed in direct contact with the media or other object desired to be thermally affected. To effectively promote heat transfer, the confinement surface should be constructed of a material having favorable conductive properties, such as a metal.
Specific applications in which heat exchangers have been advantageously employed include the microelectronics industry and the medical industry. For example, heat exchangers are used in connection with microelectronic circuits to dissipate the concentrations of heat produced by integrated circuit chips, microelectronic packages, and other components or hybrids thereof. In such an application, cooled forced air or cooled forced liquid can be used to reduce the temperature of a heat sink located adjacent to the circuit device to be cooled. An example of a heat exchanger used within the medical field is a thermal blanket used to either warm or cool patients.
Fluid transport by a conduit or other device in a heat exchanger to effect heat transfer may be characterized based on the mechanism that causes flow within the conduit or device. Where fluid transport pertains to a nonspontaneous fluid flow regime where the fluid flow results, for the most part, from an external force applied to the device, such fluid transport is considered active. In active transport, fluid flow is maintained through a device by means of a potential imposed over the flow field. This potential results from a pressure differential or concentration gradient, such as can be created using a vacuum source or a pump. Regardless of the mechanism, in active fluid transport it is a potential that motivates fluid flow through a device. A catheter that is attached to a vacuum source to draw liquid through the device is a well-known example of an active fluid transport device.
On the other hand, where the fluid transport pertains to a spontaneous flow regime where the fluid movement stems from a property inherent to the transport device, the fluid transport is considered passive. An example of spontaneous fluid transport is a sponge absorbing water. In the case of a sponge, it is the capillary geometry and surface energy of the sponge that allows water to be taken up and transported through the sponge. In passive transport, no external potential is required to motivate fluid flow through a device. A passive fluid transport device commonly used in medical procedures is an absorbent pad.
The present invention is directed to heat exchangers utilizing active fluid transport. The design of active fluid transport devices in general depends largely on the specific application to which it is to be applied. Specifically, fluid transport devices are designed based upon the volume, rate and dimensions of the particular application. This is particularly evident in active fluid transport heat exchangers, which are often required to be used in a specialized environment involving complex geometries. Moreover, the manner by which the fluid is introduced into the fluid transport device affects its design. For example, where fluid flow is between a first and second manifold, as is often the case with heat exchangers, one or multiple discrete paths can be defined between the manifolds.
In particular, in an active fluid transport heat exchanger, it is often desirable to control the fluid flow path. In one sense, the fluid flow path can be controlled for the purpose of running a particular fluid nearby an object or another fluid to remove heat from or to transfer heat to the object or other fluid in a specific application. In another sense, control of the fluid flow path can be desirable so that fluid flows according to specific flow characteristics. That is, fluid flow may be facilitated simply through a single conduit, between layers, or by way of plural channels. The fluid transport flow path may be defined by multiple discrete channels to control the fluid flow so as to, for example, minimize crossover or mixing between the discrete fluid channels. Heat exchange devices utilizing active fluid transport are also designed based upon the desired rate of heat transfer, which affects the volume and rate of the fluid flow through the heat exchanger, and on the dimensions of the heat exchanger.
Rigid heat exchangers having discrete microchannels are described in each of U.S. Pat. No. 5,527,588 to Camarda et al., U.S. Pat. No. 5,317,805 to Hoopman et al. (the '805 patent), and U.S. Pat. No. 5,249,358 to Tousignant et al. In each case, a microchanneled heat exchanger is produced by material deposition (such as by electroplating) about a sacrificial core, which is later removed to form the microchannels. In Camarda, the filaments are removed after deposition to form tubular passageways into which a working fluid is sealed. In the '805 patent to Hoopman et al, a heat exchanger comprising a first and second manifolds connected by a plurality of discrete microchannels is described. Similarly, U.S. Pat. No. 5,070,606 to Hoopman et al. describes a rigid apparatus having microchannels that can be used as a heat exchanger. The rigid microchanneled heat exchanger is made by forming a solid body about an arrangement of fibers that are subsequently removed to leave microchannels within the solid formed body. A heat exchanger is also described in U.S. Pat. No. 4,871,623 to Hoopman et al. The heat exchanger provides a plurality of elongated enclosed electroformed channels that are formed by electrodepositing material on a mandrel having a plurality of elongated ridges. Material is deposited on the edges of the ridges at a faster rate than on the inner surfaces of the ridges to envelope grooves and thus create a solid body having microchannels. Rigid heat exchangers are also known having a series of micropatterned metal platelets that are stacked together. Rectangular channels (as seen in cross section) are defined by milling channels into the surfaces of the metal platelets by microtooling.